1. Field of the Invention
The present invention relates to a transmitter using a vertical bipolar junction transistor (BJT), and more particularly to a transmitter using a BJT which can improve the junction characteristic between elements and solve a direct current (DC) offset problem by replacing at least a part of circuit elements with vertical BJTs.
2. Description of the Related Art
Currently, since complementary metal-oxide-Semiconductor (CMOS) technology has a high degree of integration as the chip size decreases, CMOS is the best technology that is used to implement a System-on-Chip (SoC). Better radio frequency (RF) performance is obtained as the chip size becomes smaller.
A direct conversion transmitter as well as a direct conversion receiver facilitates a digital circuit implementation, and particularly has a structure suitable for manufacturing a single chip using a CMOS process.
FIG. 1 is a schematic block diagram illustrating the construction of an RF direct conversion transmitter.
As illustrated in FIG. 1, the RF direct conversion transmitter includes a digital-to-analog converter 10, a baseband analog filter 20, a phase conversion element 60, a mixer 30, a drive amplifier 40, and a balloon 50. Specifically, a single or a pair of digital-to-analog converters 10, baseband analog filters 20, and mixers 30 may be provided accordingly as the direct conversion transmitter refers to a scalar or a vector, and each pair may be arranged in parallel. FIG. 1 illustrates the construction of a direct conversion transmitter that refers to a vector.
Referring to FIG. 1, a pair of digital-to-analog converters 10 are arranged in parallel, receive digital signals, i.e., I signal and Q signal, and convert them into analog signals, respectively.
A pair of baseband analog filters 20 are also arranged in parallel, perform filtering of a specific band of the I signal and the Q signal which passed through the digital-to-analog converters 10, and pass only baseband signals.
The phase conversion element 60 receives a local oscillation signal from a voltage controlled oscillator (VCO), and outputs an in-phase local oscillation signal and a quadrature phase local oscillation signal to mixers 30.
A pair of mixers 30 arranged in parallel mix the I signal or Q signal provided from the baseband analog filters 20 with an in-phase or quadrature phase local oscillation signal, and outputs an in-phase high frequency vector signal and a quadrature phase high frequency vector signal.
The drive amplifier 40 amplifies high frequency signals output from the respective mixers 30. The balloon 50 converts a balanced signal into an unbalanced signal. A duplexer may be used as the balloon 50.
Accordingly, the transmitter converts the input I signal and Q signal into analog signals, and performs filtering of the converted analog signal into the baseband to thereby output the high frequency vector signal. Then, the transmitter performs an amplifying and unbalancing of the high frequency vector signal and outputs the processed high frequency vector signal to a receiver.
FIG. 2 is a schematic block diagram illustrating the construction of an IF direct conversion transmitter.
As shown in FIG. 2, the intermediate frequency (IF) direct conversion transmitter includes a pair of digital-to-analog converters 110, a pair of analog filters 120, a phase conversion element 160, a pair of mixers 130, an up-mixer 170, a drive amplifier 140, and a balloon 150.
Here, the digital-analog converters 110, the analog filters 120, the phase conversion element 160 and the mixers 130 have the same functions as those described above in the RF direct conversion transmitter. However, it is to be noted that the analog filters 120 perform the filtering of the I signal and the Q signal which passed through the digital-to-analog converters 110 into IF signals, and the mixers 130 mix the I signal or the Q signal provided from the analog filters 120 with a first local oscillation signal L01 having an in-phase or a quadrature phase that is provided from the phase conversion element 160, and outputs an IF in-phase vector signal and an IF quadrature phase vector signal, respectively.
The up-mixer 170 mixes the IF in-phase vector signal and the IF quadrature phase vector signal, which are outputted from the respective mixers 130, with a second high frequency local oscillation signal L02, to output a high frequency in-phase vector signal and a high frequency quadrature phase vector signal, respectively.
FIG. 3 is a circuit diagram illustrating a mixer 30, 130 or 170 that is included in the direct conversion transmitter of FIGS. 1 and 2 and implemented using MOS elements.
As illustrated in FIG. 3, the mixer 30, 130 or 170 includes an amplifying unit 220 and a mixing unit 210. The amplifying unit 220 is composed of a pair of amplifying elements connected in parallel to each other, and amplifies an input signal. The mixing unit 210 includes first switching elements N3 and N4 and second switching elements N5 and N6, which are connected in pairs to be alternately turned on and off. The first and second switching elements N3, N4, N5, and N6 perform the mixing of the input signal with the local oscillation signal LO by being alternately turned on and off to each other, and output a signal corresponding to the difference between the two signals. In the conventional direct conversion transmitter, the amplifying units N1, N2 used in the mixer 30, 130, or 170 are implemented with MOS elements, and the first and second switching elements N3, N4, N5, N6 are also implemented with MOS elements.
However, it is very difficult to implement the conventional RF and IF transmitters as integrated circuits because of the DC offset occurring due to the leakage of the local oscillator and the mismatch between I/Q circuits. Especially, in the case where the direct conversion transmitter is implemented using only the CMOS process, serious problems may occur as follows.
In the case of the direct conversion transmitter, a carrier leakage problem occurs due to the DC offset based on the mismatch between the I/Q signal paths and between differential signals on the I signal path. This leakage problem causes the loss of control of a wide-range gain of the transmitter and the deterioration of an Error Vector Magnitude (EVM) characteristics which lowers the performance of the transmitter.
A bipolar junction transistor (BJT) has an excellent matching characteristic between the elements and a very small DC offset, compared with the MOS element. Accordingly, the direct conversion transmitter in which both the CMOS and BJT elements are integrated using a BiCMOS process has been developed. The DC offset of the direct conversion transmitter that uses the BiCMOS process has been remarkably improved in comparison to that using the MOS process. However, the direct conversion transmitter using the BiCMOS process have drawbacks in that its manufacturing cost is high in comparison to the CMOS process, its development takes a very long time, and it is difficult to implement such BiCMOS process in a single chip since it seriously lowers the performance of the digital circuit in comparison to the transmitter using the CMOS process.
Meanwhile, U.S. Pat. No. 5,498,885 discloses “Modulation circuit” which adopts vertical BJTs to solve the problems occurring when only MOS elements are used. However, since a vertical or lateral BJT has very poor performance of operating frequency in comparison to the MOS, its use is limited to a DC circuit such as a band-gab reference and so on, and various problems such as the lowering of the device matching characteristic have been identified.